1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device capable of electrical writing, reading, and erasing, and a manufacturing method thereof. In particular, the present invention relates to a structure of a floating gate in the nonvolatile semiconductor memory device.
2. Description of the Related Art
The market has been expanding for nonvolatile memories in which data can be electrically rewritten and data can be kept stored even after the power is turned off. Features of a nonvolatile memory are that its structure is similar to that of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a region capable of accumulating charges for a long period of time is provided over a channel formation region. This charge accumulation region which is formed over an insulating layer and is isolated and separated from the surround is also called a floating gate. Since the floating gate is surrounded with an insulator to be electrically isolated from the surround, the floating gate has a characteristic of holding charges after the charges are injected into the floating gate. Over the floating gate, a gate electrode called a control gate is further provided with an insulating layer interposed therebetween. The control gate is distinguished from the floating gate in that a predetermined voltage is applied when data is read or written.
A so-called floating-gate-type nonvolatile memory having such a structure has a mechanism in which data is stored by electrically controlling charge injection into and charge ejection from the floating gate. Specifically, the charge injection into and charge ejection from the floating gate are performed by applying a high voltage between a semiconductor layer for forming the channel formation region and the control gate. It is said that, at this time, a Fowler-Nordheim type (F-N type) tunnel current (in the case of an NAND type) or a hot electron (in the case of an NOR type) flows into the insulating layer over the channel formation region. For this reason, this insulating layer is also called a tunnel insulating layer.
The floating-gate-type nonvolatile memory is required to have a characteristic of holding charges accumulated in the floating gate for ten years or more of time in order to assure the reliability. Therefore, as for the tunnel insulating layer, it is required to form at a thickness with which the tunnel current can flow and have a high insulating property so as not to leak charges.
In addition, the floating gate formed over the tunnel insulating layer has been formed of silicon which is the same semiconductor material as that of the semiconductor for forming the channel formation region. For example, a method of forming the floating gate by using polycrystalline silicon has been spread, such as a floating gate formed by stacking a polysilicon film with a thickness of 400 nm (see page 7 and FIG. 7 in Japanese Published Patent Application No. 2000-58685)